Merge branch 'kk/merge-base-exhaustion'

The merge-base computation has been optimized by stopping the walk
early when one side's exclusive commits in the queue are exhausted,
yielding significant speedups for queries with one-sided histories.

* kk/merge-base-exhaustion:
  commit-reach: remove commit-date ordering fallback
  commit-reach: move min_generation check into paint_queue_get()
  commit-reach: terminate merge-base walk when one paint side is exhausted
  commit-reach: introduce struct paint_state with per-side counters
  t6600: add clock-skew topologies and step counts for edge cases
  commit-reach: add trace2 instrumentation to paint_down_to_common()
  t6099: add side-exhaustion regression test
  t6600: add test cases for side-exhaustion edge cases
  test-lib-functions: improve diagnostic output for trace2 data assertions
  Documentation/technical: add paint-down-to-common doc

Documentation/Makefile

@@ -129,6 +129,7 @@ TECH_DOCS += technical/long-running-process-protocol
 TECH_DOCS += technical/multi-pack-index
 TECH_DOCS += technical/packfile-uri
 TECH_DOCS += technical/pack-heuristics
+TECH_DOCS += technical/paint-down-to-common
 TECH_DOCS += technical/parallel-checkout
 TECH_DOCS += technical/partial-clone
 TECH_DOCS += technical/platform-support

Documentation/technical/meson.build

@@ -18,6 +18,7 @@ articles = [
   'multi-pack-index.adoc',
   'packfile-uri.adoc',
   'pack-heuristics.adoc',
+  'paint-down-to-common.adoc',
   'parallel-checkout.adoc',
   'partial-clone.adoc',
   'platform-support.adoc',

Documentation/technical/paint-down-to-common.adoc

@@ -0,0 +1,155 @@
+Merge-Base Computation and paint_down_to_common()
+==================================================
+
+The function `paint_down_to_common()` in `commit-reach.c` computes merge
+bases by walking the commit graph backwards from two sets of tips and
+finding where their ancestry meets.
+
+Use cases
+---------
+
+Computing merge bases is used in two different ways:
+
+ 1. *Finding all merge bases* (`merge-base --all`, `merge-tree`,
+    `merge`, `rebase`). A merge base is a common ancestor that is
+    not itself an ancestor of another common ancestor.
+
+ 2. *Ancestry checks* (`in_merge_bases`, used by `merge-base
+    --is-ancestor`, `branch -d`, `fetch`). These ask: "is commit A
+    an ancestor of commit B?" If a common ancestor equals one of the
+    inputs, that input is necessarily the only merge base -- no other
+    common ancestor can be both as recent and not an ancestor of it.
+
+Both use cases share the same algorithm and implementation.
+
+Algorithm
+---------
+
+Given a commit `one` and a set of commits `twos[]`, the walk paints
+commits with two colors:
+
+  - PARENT1: reachable from `one`
+  - PARENT2: reachable from any commit in `twos[]`
+
+The walk uses a priority queue ordered by generation number
+(highest first), breaking ties by commit date. Each step dequeues
+the highest-priority commit and propagates its paint flags to its
+parents, enqueuing any parent that gained new flags. When a
+commit receives both PARENT1 and PARENT2, it is a merge-base
+candidate. A candidate gains the STALE flag so its ancestors
+propagate staleness -- any deeper common ancestor is necessarily
+redundant.
+
+[[generation-regions]]
+Topologically ordered and unordered generation regions
+------------------------------------------------------
+
+Commits fall into two regions based on whether their generation
+numbers provide a topological ordering guarantee:
+
+....
+    +------------------------------------------+
+    |          Unordered region                |
+    |  generation = INFINITY or V1_MAX         |
+    |  queue order: heuristic (commit date)    |
+    +------------------------------------------+
+                      |
+                      v
+    +------------------------------------------+
+    |          Ordered region                  |
+    |  generation = finite, unsaturated        |
+    |  queue order: topological                |
+    +------------------------------------------+
+....
+
+In the ordered region, a child's generation is strictly greater
+than its parent's. Same-generation commits are necessarily
+independent, so the queue always processes children before
+their parents.
+
+In the unordered region, parent-child pairs can share the same
+generation number, so topological order is not guaranteed. The
+queue uses commit-date as a heuristic, which typically produces
+a reasonable traversal order but may process a parent before
+its child.
+
+Commits not in the commit-graph have generation INFINITY; v1
+commit-graphs saturate at V1_MAX. Both place commits in the
+unordered region. Any optimization that depends on generation
+ordering must account for this saturation boundary. The early
+exit gates compare against a topological ceiling --
+`GENERATION_NUMBER_V1_MAX` for v1 graphs and
+`GENERATION_NUMBER_INFINITY` for v2 graphs -- so that saturated
+commits are treated as unordered.
+
+With generation ordering, values in the unordered region exceed
+those in the ordered region. The walk may therefore transition
+from the unordered region into the ordered region, but never in
+the reverse direction. Without a commit-graph, every commit has INFINITY
+and the walk operates entirely in the unordered region.
+
+In the ordered region, paint on a dequeued commit is final --
+no future step can add flags to it. In the unordered region,
+a dequeued commit may later gain additional paint. Paint flags
+are only added, never removed, bounding the number of
+re-enqueues per commit.
+
+Termination
+-----------
+
+The walk tracks the number of commits of each type in the queue
+(PARENT1-only, PARENT2-only, pending merge-base). The main loop
+ends when one of the following conditions holds:
+
+  1. The queue is empty.
+  2. The queue contains only stale entries.
+  3. Generation cutoff: the dequeued commit's generation is below
+     a caller-supplied `min_generation` threshold.
+  4. Single result: the caller only needs one merge base, one has
+     been found, and the walk has entered the ordered region.
+  5. Side exhaustion: no pure PARENT1 or pure PARENT2 commits
+     remain in the queue, no pending merge-base candidates exist,
+     and the walk has entered the ordered region.
+
+Stale entry condition
+~~~~~~~~~~~~~~~~~~~~~
+Once all queued entries are stale, no new merge-base candidates can
+be discovered -- that requires at least one non-stale commit from
+each side meeting. Continuing the walk could still invalidate
+existing candidates by proving one is an ancestor of another, but
+`remove_redundant()` handles that as a post-processing step, so it
+is safe to exit early.
+
+Side-exhaustion condition
+~~~~~~~~~~~~~~~~~~~~~~~~~
+A new merge-base requires commits from both sides to meet. When one
+side's exclusive counter reaches zero and there are no pending
+merge-base candidates, no future traversal step can produce a new
+candidate. This optimization only activates in the ordered region,
+where paint flags are final at visit time; in the unordered region,
+a side that appears exhausted could reappear through late paint
+propagation.
+
+Generation cutoff
+~~~~~~~~~~~~~~~~~
+Some callers (notably `remove_redundant()`) supply a `min_generation`
+threshold equal to the minimum generation of the input commits.
+These callers only need to determine reachability among the inputs,
+not find deep merge bases, so the walk can safely terminate when it
+dequeues a commit below this threshold.
+
+Single result
+~~~~~~~~~~~~~
+When only one merge base is needed and the walk is in the
+ordered region with generation ordering, the first candidate
+found is necessarily the highest-generation common ancestor.
+No remaining commit in the queue can be a descendant of this
+candidate (generation ordering guarantees children are visited
+first), so it cannot be redundant and the walk can stop
+immediately.
+
+Related documentation
+---------------------
+
+  - `Documentation/technical/commit-graph.adoc` -- generation numbers
+    and the reachability closure property.

commit-reach.c

@@ -11,6 +11,7 @@
 #include "tag.h"
 #include "commit-reach.h"
 #include "ewah/ewok.h"
+#include "trace2.h"
 
 /* Remember to update object flag allocation in object.h */
 #define PARENT1		(1u<<16)
@@ -78,25 +79,111 @@ static void clear_nonstale_queue(struct nonstale_queue *queue)
 	queue->max_nonstale = NULL;
 }
 
-static void nonstale_queue_put_dedup(struct nonstale_queue *queue,
+/*
-				     struct commit *c)
+ * Priority queue with per-side commit counters for paint_down_to_common().
+ * Each non-stale queued commit occupies exactly one bucket: PARENT1-only,
+ * PARENT2-only, or both (a pending merge-base candidate).
+ */
+struct paint_state {
+	struct prio_queue queue;
+	size_t parent1_count;
+	size_t parent2_count;
+	size_t mb_candidate_count;
+	timestamp_t min_generation;
+	timestamp_t last_gen;
+	timestamp_t topo_ceiling;
+};
+
+static void paint_count_update(struct paint_state *state,
+			       unsigned flags, int delta)
 {
-	if (c->object.flags & ENQUEUED)
+	switch (flags & (PARENT1 | PARENT2 | STALE)) {
-		return;
+	case PARENT1:
-	c->object.flags |= ENQUEUED;
+		state->parent1_count += delta;
-	nonstale_queue_put(queue, c);
+		break;
+
+	case PARENT2:
+		state->parent2_count += delta;
+		break;
+
+	case PARENT1 | PARENT2:
+		state->mb_candidate_count += delta;
+		break;
+
+	case PARENT1 | PARENT2 | STALE:
+		break;
+
+	default:
+		BUG("unexpected paint state");
+	}
 }
 
-static struct commit *nonstale_queue_get_dedup(struct nonstale_queue *queue)
+static void paint_queue_put(struct paint_state *state,
+			    struct commit *c, unsigned add_flags)
 {
-	struct commit *commit = nonstale_queue_get(queue);
+	unsigned old_flags = c->object.flags;
+	c->object.flags |= add_flags;
 
-	if (commit)
+	if (old_flags & ENQUEUED) {
-		commit->object.flags &= ~ENQUEUED;
+		paint_count_update(state, old_flags, -1);
+		paint_count_update(state, c->object.flags, 1);
+	} else {
+		c->object.flags |= ENQUEUED;
+		prio_queue_put(&state->queue, c);
+		paint_count_update(state, c->object.flags, 1);
+	}
+}
+
+/*
+ * Dequeue the next commit for the paint walk, or return NULL when
+ * no more merge bases can be discovered.
+ */
+static struct commit *paint_queue_get(struct paint_state *state)
+{
+	struct commit *commit = prio_queue_get(&state->queue);
+	timestamp_t generation;
+
+	if (!commit)
+		return NULL;
+
+	commit->object.flags &= ~ENQUEUED;
+	generation = commit_graph_generation(commit);
+
+	if (state->min_generation && generation > state->last_gen)
+		BUG("bad generation skip %"PRItime" > %"PRItime" at %s",
+		    generation, state->last_gen,
+		    oid_to_hex(&commit->object.oid));
+	state->last_gen = generation;
+
+	/* generation cutoff */
+	if (generation < state->min_generation)
+		return NULL;
+
+	/*
+	 * Check exit condition before decrementing: the counters
+	 * still include this commit, so the last non-stale commit
+	 * sees a non-zero count and is returned for processing.
+	 */
+	if (!state->mb_candidate_count) {
+		/* only stale entries remain */
+		if (!state->parent1_count && !state->parent2_count)
+			return NULL;
+
+		/* one side is exhausted */
+		if ((!state->parent1_count || !state->parent2_count) &&
+		    generation < state->topo_ceiling)
+			return NULL;
+	}
+
+	paint_count_update(state, commit->object.flags, -1);
 	return commit;
 }
 
-/* all input commits in one and twos[] must have been parsed! */
+/*
+ * See Documentation/technical/paint-down-to-common.adoc
+ *
+ * All input commits in one and twos[] must have been parsed!
+ */
 static int paint_down_to_common(struct repository *r,
 				struct commit *one, int n,
 				struct commit **twos,
@@ -104,45 +191,40 @@ static int paint_down_to_common(struct repository *r,
 				enum merge_base_flags mb_flags,
 				struct commit_list **result)
 {
-	struct nonstale_queue queue = {
+	/*
-		{ compare_commits_by_gen_then_commit_date }
+	 * Generation ordering is required for the side-exhaustion and
+	 * single-result early exits, which rely on topological traversal
+	 * order (children visited before parents) in the ordered region.
+	 */
+	struct paint_state state = {
+		.queue = { compare_commits_by_gen_then_commit_date }
 	};
+	struct commit *commit;
 	int i;
-	int gen_ordered = 1;
+	int steps = 0;
-	timestamp_t last_gen = GENERATION_NUMBER_INFINITY;
 	struct commit_list **tail = result;
 
-	if (!min_generation && !corrected_commit_dates_enabled(r)) {
+	state.min_generation = min_generation;
-		queue.pq.compare = compare_commits_by_commit_date;
+	state.last_gen = GENERATION_NUMBER_INFINITY;
-		gen_ordered = 0;
+	state.topo_ceiling = corrected_commit_dates_enabled(r)
-	}
+		? GENERATION_NUMBER_INFINITY
+		: GENERATION_NUMBER_V1_MAX;
+
 
 	one->object.flags |= PARENT1;
 	if (!n) {
 		commit_list_append(one, result);
 		return 0;
 	}
-	nonstale_queue_put_dedup(&queue, one);
+	paint_queue_put(&state, one, 0);
 
-	for (i = 0; i < n; i++) {
+	for (i = 0; i < n; i++)
-		twos[i]->object.flags |= PARENT2;
+		paint_queue_put(&state, twos[i], PARENT2);
-		nonstale_queue_put_dedup(&queue, twos[i]);
-	}
 
-	while (queue.max_nonstale) {
+	while ((commit = paint_queue_get(&state))) {
-		struct commit *commit = nonstale_queue_get_dedup(&queue);
 		struct commit_list *parents;
 		int flags;
-		timestamp_t generation = commit_graph_generation(commit);
+		steps++;
-
-		if (min_generation && generation > last_gen)
-			BUG("bad generation skip %"PRItime" > %"PRItime" at %s",
-			    generation, last_gen,
-			    oid_to_hex(&commit->object.oid));
-		last_gen = generation;
-
-		if (generation < min_generation)
-			break;
 
 		flags = commit->object.flags & (PARENT1 | PARENT2 | STALE);
 		if (flags == (PARENT1 | PARENT2)) {
@@ -155,8 +237,7 @@ static int paint_down_to_common(struct repository *r,
 				 * descendant of this one.
 				 */
 				if (!(mb_flags & MERGE_BASE_FIND_ALL) &&
-				    gen_ordered &&
+				    state.last_gen < state.topo_ceiling)
-				    generation < GENERATION_NUMBER_INFINITY)
 					break;
 			}
 			/* Mark parents of a found merge stale */
@@ -169,7 +250,7 @@ static int paint_down_to_common(struct repository *r,
 			if ((p->object.flags & flags) == flags)
 				continue;
 			if (repo_parse_commit(r, p)) {
-				clear_nonstale_queue(&queue);
+				clear_prio_queue(&state.queue);
 				commit_list_free(*result);
 				*result = NULL;
 				/*
@@ -184,12 +265,13 @@ static int paint_down_to_common(struct repository *r,
 				return error(_("could not parse commit %s"),
 					     oid_to_hex(&p->object.oid));
 			}
-			p->object.flags |= flags;
+			paint_queue_put(&state, p, flags);
-			nonstale_queue_put_dedup(&queue, p);
 		}
 	}
 
-	clear_nonstale_queue(&queue);
+	clear_prio_queue(&state.queue);
+	trace2_data_intmax("paint_down_to_common", r,
+			   "steps", steps);
 	commit_list_sort_by_date(result);
 	return 0;
 }

t/meson.build

@@ -796,6 +796,7 @@ integration_tests = [
   't6041-bisect-submodule.sh',
   't6050-replace.sh',
   't6060-merge-index.sh',
+  't6099-merge-base-side-exhaustion.sh',
   't6100-rev-list-in-order.sh',
   't6101-rev-parse-parents.sh',
   't6102-rev-list-unexpected-objects.sh',

t/t6099-merge-base-side-exhaustion.sh

@@ -0,0 +1,82 @@
+#!/bin/sh
+
+test_description='merge-base with ancestor among merge-base candidates
+
+Test that merge-base --all correctly handles cases where
+multiple merge-base candidates exist and one is an ancestor
+of another. The side-exhaustion optimization in
+paint_down_to_common may exit before STALE propagation
+removes the ancestor, but remove_redundant catches it.
+
+Graph shape (parents are below children):
+
+   A ----- X
+   |\     /|
+   | B---/ |
+   |  \    |
+   e2  \   f2
+   |   |   |
+   e1  d1  f1
+    \  |  /
+     \ | /
+      \|/
+       C
+
+A and X are the two tips.
+B and C are both reachable from A and X.
+B reaches C through d1.
+Only B should appear in merge-base --all output.
+'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+test_expect_success 'setup ancestor merge-base candidate' '
+	test_commit C &&
+
+	git checkout -b d-chain HEAD &&
+	test_commit d1 &&
+	test_commit B &&
+
+	git checkout -b e-path C &&
+	test_commit e1 &&
+	test_commit e2 &&
+
+	git checkout -b f-path C &&
+	test_commit f1 &&
+	test_commit f2 &&
+
+	git checkout -b branch-A e-path &&
+	test_merge A B &&
+
+	git checkout -b branch-X f-path &&
+	test_merge X B &&
+
+	git commit-graph write --reachable
+'
+
+test_expect_success 'merge-base --all excludes ancestor candidate' '
+	git rev-parse B >expected &&
+	git merge-base --all A X >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'merge-base (single) finds shallowest' '
+	git rev-parse B >expected &&
+	git merge-base A X >actual &&
+	test_cmp expected actual
+'
+
+# Without commit-graph: generation numbers are INFINITY,
+# side-exhaustion optimization does not fire.
+test_expect_success 'merge-base --all without commit-graph' '
+	rm -f .git/objects/info/commit-graph &&
+	git rev-parse B >expected &&
+	git merge-base --all A X >actual &&
+	test_cmp expected actual
+'
+
+test_done

t/t6600-test-reach.sh

@@ -85,6 +85,103 @@ test_expect_success 'setup' '
 	git branch -f skew-P2 "$skew_P2" &&
 	git tag skew-M2 "$skew_M2" &&
 
+	# Build a small side topology to exercise the (PARENT1|PARENT2) ->
+	# (PARENT1|PARENT2|STALE) transition in paint_down_to_common(); the
+	# 10x10 grid above does not exercise it because no merge-base candidate
+	# there is a descendant of another, so STALE never reaches a
+	# still-pending candidate.
+	#
+	#       ps-X
+	#       /|\
+	#      / | \
+	#   ps-Z ps-B ps-W
+	#     |  / \  |
+	#     | /   \ |
+	#     |/     \|
+	#   ps-T1   ps-T2
+	#
+	# where ps-T1=merge(ps-Z,ps-B), ps-T2=merge(ps-W,ps-B), so
+	# merge-base(ps-T1,ps-T2) = ps-B.  During the walk, ps-X transitions
+	# to (PARENT1|PARENT2) via ps-Z and ps-W before ps-B is dequeued;
+	# then the STALE-walk from ps-B transitions ps-X to
+	# (PARENT1|PARENT2|STALE).
+	git checkout --orphan ps-orphan &&
+	test_commit ps-X &&
+	git checkout -b ps-B-br ps-X && test_commit ps-B &&
+	git checkout -b ps-Z-br ps-X && test_commit ps-Z &&
+	git checkout -b ps-W-br ps-X && test_commit ps-W &&
+	git checkout -b ps-T1 ps-Z &&
+	git merge --no-ff -m ps-T1 ps-B &&
+	git checkout -b ps-T2 ps-W &&
+	git merge --no-ff -m ps-T2 ps-B &&
+
+	# Build a side topology that lives entirely outside the half
+	# commit-graph and has non-monotonic commit dates, to exercise the
+	# INFINITY-gate in paint_down_to_common.  With both tips outside
+	# the graph, generation is INFINITY and the queue falls back to
+	# commit-date order, which here is non-monotonic.
+	#
+	#   pi-X (date 500, PARENT1 tip) --> pi-P, pi-D
+	#   pi-D (date 480) --> pi-C
+	#   pi-C (date 200) --> pi-B
+	#   pi-B (date 100, PARENT2 tip) --> pi-P
+	#   pi-P (date 450, root)
+	#
+	# merge-base(pi-X, pi-B) = pi-B (it is an ancestor of pi-X and is
+	# itself one of the queried tips).
+	git checkout --orphan pi-orphan &&
+	test_commit --date "@450 +0000" pi-P &&
+	test_commit --date "@100 +0000" pi-B &&
+	test_commit --date "@200 +0000" pi-C &&
+	test_commit --date "@480 +0000" pi-D &&
+	GIT_AUTHOR_DATE="@500 +0000" GIT_COMMITTER_DATE="@500 +0000" \
+		git commit-tree -p pi-D -p pi-P -m pi-X pi-D^{tree} >pi-X-oid &&
+	pi_x="$(cat pi-X-oid)" &&
+	git branch -f pi-X-br "$pi_x" &&
+	git tag pi-X "$pi_x" &&
+
+	# Clock-skew topology for side-exhaustion testing.
+	# D is the correct merge base but has a higher committer date
+	# than C (its child).  With date ordering, D would be dequeued
+	# before C, causing side-exhaustion to fire too early.
+	# Generation ordering prevents this by visiting children
+	# before parents regardless of dates.
+	#
+	#   se-A (date 7000) --> se-C (date 3000) --> se-D (date 5000) --> se-root (date 4000)
+	#   se-B (date 6000) --> se-D
+	#
+	se_root=$(skew_commit 4000 se-root) &&
+	se_D=$(skew_commit 5000 se-D -p "$se_root") &&
+	se_C=$(skew_commit 3000 se-C -p "$se_D") &&
+	se_A=$(skew_commit 7000 se-A -p "$se_C") &&
+	se_B=$(skew_commit 6000 se-B -p "$se_D") &&
+	git branch -f se-A "$se_A" &&
+	git branch -f se-B "$se_B" &&
+	git tag se-D "$se_D" &&
+
+	# Clock-skew topology with redundant ancestor for
+	# side-exhaustion testing.  MB1 is the correct merge base;
+	# MB2 is its parent.  A reaches MB2 via E (high date) and
+	# MB1 via C (low date).  B reaches MB1 via D.  With date
+	# ordering, side-exhaustion would fire before C is dequeued,
+	# missing MB1.  Generation ordering ensures both are found.
+	#
+	#   se2-A (date 8000) --> se2-C (date 2000) --> se2-MB1 (date 5000) --> se2-MB2 (date 4000) --> se2-root (date 1000)
+	#   se2-A              --> se2-E (date 6500) --> se2-MB2
+	#   se2-B (date 7000) --> se2-D (date 6000) --> se2-MB1
+	#
+	se2_root=$(skew_commit 1000 se2-root) &&
+	se2_MB2=$(skew_commit 4000 se2-MB2 -p "$se2_root") &&
+	se2_MB1=$(skew_commit 5000 se2-MB1 -p "$se2_MB2") &&
+	se2_C=$(skew_commit 2000 se2-C -p "$se2_MB1") &&
+	se2_D=$(skew_commit 6000 se2-D -p "$se2_MB1") &&
+	se2_E=$(skew_commit 6500 se2-E -p "$se2_MB2") &&
+	se2_A=$(skew_commit 8000 se2-A -p "$se2_C" -p "$se2_E") &&
+	se2_B=$(skew_commit 7000 se2-B -p "$se2_D") &&
+	git branch -f se2-A "$se2_A" &&
+	git branch -f se2-B "$se2_B" &&
+	git tag se2-MB1 "$se2_MB1" &&
+
 	git commit-graph write --reachable &&
 	mv .git/objects/info/commit-graph commit-graph-full &&
 	chmod u+w commit-graph-full &&
@@ -98,24 +195,34 @@ test_expect_success 'setup' '
 '
 
 run_all_modes () {
-	test_when_finished rm -rf .git/objects/info/commit-graph &&
+	graph=.git/objects/info/commit-graph &&
-	"$@" <input >actual &&
+	test_when_finished rm -rf "$graph" "${graph}s" &&
-	test_cmp expect actual &&
+	rm -f trace-mode-*.txt &&
-	cp commit-graph-full .git/objects/info/commit-graph &&
+
-	"$@" <input >actual &&
+	for mode in none full half no-gdat
-	test_cmp expect actual &&
+	do
-	cp commit-graph-half .git/objects/info/commit-graph &&
+		rm -rf "$graph" "${graph}s" &&
-	"$@" <input >actual &&
+		cp "commit-graph-${mode}" "$graph" 2>/dev/null ||
-	test_cmp expect actual &&
+		true &&
-	cp commit-graph-no-gdat .git/objects/info/commit-graph &&
+		GIT_TRACE2_EVENT="$(pwd)/trace-mode-${mode}.txt" \
-	"$@" <input >actual &&
+			"$@" <input >actual &&
-	test_cmp expect actual
+		test_cmp expect actual || return 1
+	done
 }
 
 test_all_modes () {
 	run_all_modes test-tool reach "$@"
 }
 
+test_paint_down_steps () {
+	for mode in none full half no-gdat
+	do
+		test_trace2_data_singular paint_down_to_common steps "$1" \
+			"mode=$mode" <"trace-mode-${mode}.txt" || return 1
+		shift
+	done
+}
+
 test_expect_success 'ref_newer:miss' '
 	cat >input <<-\EOF &&
 	A:commit-5-7
@@ -182,6 +289,17 @@ test_expect_success 'in_merge_bases_many:miss-heuristic' '
 	test_all_modes in_merge_bases_many
 '
 
+test_expect_success 'in_merge_bases_many:self' '
+	cat >input <<-\EOF &&
+	A:commit-6-8
+	X:commit-5-9
+	X:commit-6-8
+	EOF
+	echo "in_merge_bases_many(A,X):1" >expect &&
+	test_all_modes in_merge_bases_many &&
+	test_paint_down_steps 45 1 25 1
+'
+
 test_expect_success 'is_descendant_of:hit' '
 	cat >input <<-\EOF &&
 	A:commit-5-7
@@ -219,6 +337,105 @@ test_expect_success 'get_merge_bases_many' '
 	test_all_modes get_merge_bases_many
 '
 
+test_expect_success 'get_merge_bases_many:duplicate-twos' '
+	cat >input <<-\EOF &&
+	A:commit-5-7
+	X:commit-4-8
+	X:commit-4-8
+	X:commit-6-6
+	X:commit-6-6
+	X:commit-8-3
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse commit-5-6 \
+			      commit-4-7 | sort
+	} >expect &&
+	test_all_modes get_merge_bases_many
+'
+
+test_expect_success 'get_merge_bases_many:pending-stale' '
+	# Exercises the (PARENT1|PARENT2) -> (...|STALE) transition path in
+	# paint_down_to_common().  See the topology comment in the setup test.
+	cat >input <<-\EOF &&
+	A:ps-T1
+	X:ps-T2
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse ps-B
+	} >expect &&
+	test_all_modes get_merge_bases_many &&
+	test_paint_down_steps 5 5 5 5
+'
+
+test_expect_success 'get_merge_bases_many:infinity-both-sides' '
+	# Exercises the push-time INFINITY-gate in paint_down_to_common().  See
+	# the pi-* topology comment in the setup test.
+	cat >input <<-\EOF &&
+	A:pi-X
+	X:pi-B
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse pi-B
+	} >expect &&
+	test_all_modes get_merge_bases_many &&
+	test_paint_down_steps 5 4 5 4
+'
+
+test_expect_success 'setup mixed finite/INFINITY topology' '
+	# Create a commit outside all saved commit-graph files so it always
+	# has INFINITY generation, while its parent (ps-X) is in the graph
+	# with a finite generation. Use the ps-* orphan topology so we do
+	# not pollute the grid-based rev-list tests.
+	git checkout ps-X &&
+	test_env GIT_TEST_COMMIT_GRAPH= test_commit pm-INF
+'
+
+test_expect_success 'get_merge_bases_many:mixed-finite-infinity' '
+	# One tip (pm-INF) is outside the commit-graph with INFINITY
+	# generation; the other (ps-B) is in the graph with finite
+	# generation. The walk starts in the INFINITY region and crosses
+	# into the finite region where side-exhaustion can fire.
+	cat >input <<-\EOF &&
+	A:pm-INF
+	X:ps-B
+	EOF
+	{
+		echo "get_merge_bases_many(A,X):" &&
+		git rev-parse ps-X
+	} >expect &&
+	test_all_modes get_merge_bases_many &&
+	test_paint_down_steps 3 3 3 3
+'
+
+test_expect_success 'merge-base --all commit-walk steps' '
+	>input &&
+	git rev-parse commit-9-1 >expect &&
+	run_all_modes git merge-base --all commit-9-9 commit-9-1 &&
+	test_paint_down_steps 81 9 57 37
+'
+
+test_expect_success 'merge-base --all with clock skew (side-exhaustion)' '
+	# Verify that the merge base is computed correctly even
+	# when commits have non-monotonic commit dates.
+	>input &&
+	git rev-parse se-D >expect &&
+	run_all_modes git merge-base --all se-A se-B &&
+	test_paint_down_steps 6 4 6 4
+'
+
+test_expect_success 'merge-base --all with clock skew and redundant ancestor (side-exhaustion)' '
+	# Verify that the correct merge base is found even when
+	# non-monotonic commit dates could cause a redundant
+	# ancestor to be visited first.
+	>input &&
+	git rev-parse se2-MB1 >expect &&
+	run_all_modes git merge-base --all se2-A se2-B &&
+	test_paint_down_steps 8 6 8 6
+'
+
 test_expect_success 'reduce_heads' '
 	cat >input <<-\EOF &&
 	X:commit-1-10

t/test-lib-functions.sh

@@ -2004,6 +2004,41 @@ test_trace2_data () {
 	grep -e '"category":"'"$1"'","key":"'"$2"'","value":"'"$3"'"'
 }
 
+# Check that the given trace2 data event has the expected value and
+# appears exactly once.  Produces a diagnostic on failure.
+#
+#	test_trace2_data_singular <category> <key> <value> [<label>]
+test_trace2_data_singular () {
+	local category="$1" key="$2" expect_val="$3"
+	local label_suffix="${4:+ [$4]}"
+	local kv_pattern='"category":"'"$category"'","key":"'"$key"'","value":"\([^"]*\)"'
+	local actual
+
+	actual=$(sed -n "s|.*${kv_pattern}.*|\1|p") &&
+
+	if test -z "$actual"
+	then
+		echo >&4 "error: trace2 data '$category/$key'$label_suffix not found"
+		return 1
+	fi &&
+
+	case "$actual" in
+	*"$LF"*)
+		echo >&4 "error: trace2 data '$category/$key'$label_suffix has multiple entries, expected 1"
+		printf '%s\n' "$actual" | sed 's/^/  actual:   /' >&4
+		return 1
+		;;
+	esac &&
+
+	if test "$actual" != "$expect_val"
+	then
+		echo >&4 "error: trace2 data '$category/$key'$label_suffix"
+		echo >&4 "  expected: $expect_val"
+		echo >&4 "  actual:   $actual"
+		return 1
+	fi
+}
+
 # Given a GIT_TRACE2_EVENT log over stdin, writes to stdout a list of URLs
 # sent to git-remote-https child processes.
 test_remote_https_urls() {